1. Field of the Invention
This invention relates to improvements in a rotor-type oil pump, and more particularly to such an oil pump suitable for supplying lubricating oil to a variety of oil-requiring parts in an internal combustion engine.
2. Description of the Prior Art
Hitherto a variety of rotor-type oil pumps for the purpose of supplying lubricating oil to oil-requiting parts have been proposed and put into practical use in the field of automotive internal combustion engine. An example of such a rotor-type oil pump is disclosed in Japanese Utility Model Publication No. 3-122283. This rotor-type oil pump includes a generally annular outer rotor which is rotatably disposed in a pump casing. A generally annular inner rotor is disposed eccentrically inside the outer rotor and has an external gear which is partly in mesh with the internal gear of the outer rotor. Upon driving the inner rotor, the outer rotor is rotated thereby accomplishing a pumping action in which oil is sucked and then discharged.
Each of the outer and inner rotors is formed of an iron-base sintered alloy. Additionally, the height and width of teeth, the thickness of a tooth base section and the like of the internal and external gears of outer and inner rotors are so designed that the outer and inner rotors have the generally same static breaking load so as to meet the same durability.
However, difficulties have been encountered in such a conventional oil pump arrangement in which the outer and inner rotors are designed to have the same durability. That is, in such a case, the outer rotor having a larger outer diameter has an excessive stress resistance, in which the tooth base section of the internal gear of the outer rotor is formed too thick. As a result, the oil pump is made large-sized and increased in weight while increasing a power consumed by driving the pump.